80 were sent to the US Supreme Court. What I always tell my students is that this is a place where the justices and their clerks get many arguments to present in their decision. Now a W&M (proud alum here) professor has done her research and found that between 2008 and 2013 there were 124 citations from amici briefs. Considering that there are only roughly 70 cases a year - so 420 total over the period - that means there is a one and four chance of an amici brief having its facts mentioned. Of course this doesn't forget that oft cited are also the plaintiff and defendant briefs. More here.